The present invention relates to information storage devices such as magnetic disk storage devices, optical disk storage devices, or magneto-optical disk storage devices. In particular, the present invention relates to a technology that is usefully applied to information storage devices which utilize magneto-resistive heads or optical heads.
Due to the recent increase in the amount of data processed by information storage devices, there is an increase in the requirements for larger storage capacity and faster transfer rate of these devices. At the same time, there is an increase in the need for smaller size devices due to the growth of portable information processing systems.
An increase in the storage capacity of information storage devices can be realized by increasing the information recording density on the recording media. However, increase in recording density causes a decrease in the output of the transducer for reading-out the information, thereby resulting in a deteriorating signal-to-noise ratio (S/N ratio).
A partial-response system (PR equalizing system) is known as a method for compensating for the deterioration in S/N ratio of the transducer output, as mentioned in xe2x80x9cNew Coinage Dictionary for Information and Communication, ""96 editionxe2x80x9d, p501 (Oct. 9. ""95, Nikkei BP, in Japanese). The PR equalizing system is a system in which the signal is retrieved by causing some regular interference to the detected waveform. This way, data can be recorded/reproduced in narrower bandwidth than in the conventional sample detecting system. In particular, the PRML (Partial-Response Maximum Likelihood) system combined with the Viterbi decoding system in which the most likely correct data sequence is chosen from a data sequence being made correlative in advance, results in an effective method for compensating for the deterioration of S/N ratio as the data error rate is minimized.
Magneto-resistive (MR) heads which are furnished without coils for recording/reproducing have recently been adopted in magnetic recording systems as explained, for example, in xe2x80x9cElectronic Materials, Nov.""94 editionxe2x80x9d, p22-28 (Nov. 1. ""94, The Japanese Investigative Association for the Industry, in Japanese). The MR head is peculiar in that its output has no dependency on the relative velocity of the recording media because the MR head is a magnetic flux response type where reproducing noise is not generated due to absence of coils. This results in improved S/N ratio, high sensitivity signal detection, and high correspondence for high frequency recording due to its low inductance. Thus, such a transducer is suitable for high density recording and high speed reproducing. Moreover, the MR head can be easily combined with the PRML system as described in xe2x80x9cElectronic Materials, Nov. ""94 editionxe2x80x9d, p22-28, mentioned above. This combination of the above-mentioned two technologies makes it promising to obtain a magnetic disk storage with higher capacity and smaller size.
On the other hand, with respect to a system in which information is optically read-out from the recording media, optical heads in which laser diodes are used as a light source are used as reproducing transducers, as explained in xe2x80x9cInformation Processing Handbookxe2x80x9d, pp283-284 (May 30, 1989, Ohm, in Japanese), for example. The optical head reads out recorded data by irradiating laser light on the recording media.
The MR head in principle requires sense current, because information on the recording media is detected as change of resistance in the MR head. The sense current is continuously applied in a DC mode at least during the reproduction of recorded data, as explained in above mentioned xe2x80x9cElectronic Materials, Nov. ""94 editionxe2x80x9d, p22-28 or p35-40. The laser diode, used as a light source, also emits light continuously in a DC mode, as explained in above mentioned xe2x80x9cInformation Processing Handbookxe2x80x9d, pp283-284.
As mentioned above, the MR head sense current and driving current for the laser diode in the optical head are supplied in a DC mode. The present inventors have recognized that this might cause deterioration or lifetime problems in the transducers. In the conventional signal processing technology in which PRML system is combined with signal detection, the analog value of the transducer output is sampled and processed in a suitable way for the PRML system, and so, a time-continuous value is required for the transducer output. Because the transducers, such as MR head or optical head, are active elements, sense current of the MR head or driving current for the laser diode in the optical head is required. For this reason, DC current supply for sense current of the MR head or for driving current to the laser diode is indispensable, inevitably causing an electrical migration in the MR head or the laser diode. Thus, deterioration and lifetime problems may arise due to this electrical migration.
In order to meet the demand for higher information recording density, it is necessary to optimize the trade-offs between the recording density and detecting sensitivity of the transducer. While the increase in sense-current of the MR head or driving current to the laser diode may improve detecting sensitivity, it may not only increase the electrical migration, but may also cause severe deterioration of the transducers due to thermal effect caused by increase in power consumption. This could result in an even further shortened lifetime.
Accordingly, a purpose of the present invention is to prevent deterioration of the transducers such as MR heads or optical heads and to lengthen their lifetime.
Another purpose of the present invention is to improve sensitivity of the transducers such as MR heads or optical heads, without shortening their lifetime.
Yet another purpose of the present invention is to provide a technology to meet the competing needs of increased storage capacity and smaller size through increasing the recording density on the recording media by increasing in sensitivity of the transducers.
The information storage device of the present invention includes information recording media on which modulated signals that are modulated in a specified rule of data used in the information processing unit are recorded, transducers which detect the modulated signals from the information recording media, converters which convert output signals from the transducers to digital values, and a demodulator which retrieves data from digital values. This information storage device is equipped with a clock generator that generates one or a plural number of timing clocks synchronized with each sampling period for converting output signals to digital values. The transducers operate in an pulse mode in synchronism with the timing clocks.
In such an information storage device, which is equipped with a clock generator that generates one or a plural number of timing clocks in synchronism with each sampling period for converting output signals to digital values, and the transducers operate in an pulse mode in synchronism with the timing clocks, driving current to drive the transducers such as the sense current or laser current is not applied to the transducers in a DC mode but applied only during the period of operating in a pulse mode in synchronism with the timing clock. Therefore, average of the driving current is reduced, resulting in suppression of electrical migration due to the driving current, thereby extending the lifetime of the transducers and improving the reliability of the information storage device.
Since the average of the driving current is decreased to increase the transducer lifetime, the instantaneous driving current, that is applied during the period of operating in a pulse mode, can be increased. Increase of instantaneous driving current does not bring increase in power consumption of the transducer if the width of said pulse is kept small, thereby preventing shortening of lifetime of the transducer due to thermal deterioration. At the same time, the increase of instantaneous driving current, improves transducer sensitivity, resulting in improved recording density on the recording media in order to realize larger storage capacity and smaller size for the information storage device. In other words, it is possible to increase transducer sensitivity without the accompanying deterioration or shortening of the lifetime of the transducers. Accordingly, the present invention can realize a device with a larger storage capacity along and a smaller size.
The timing clock, which is in synchronism with the sampling period, can be not only one but a plural number for every sampling period. By using-a plural number of timing clocks for every sampling period, a plural number of sampled values can be obtained. By applying a technology such as a digital filter to obtain a plural number of sampled values, reliability of measured values can be improved.
The transducer mentioned above can be a magneto-resistive head or an optical head including a laser diode as a light source. In response to the requirement for a smaller sized magneto-resistive head or optical head, such elements tends to be unstable against heat. Accordingly, the present invention may be of great consequence. The applied current is a sense current for the case of a magneto-resistive head and is a driving current for the case of a laser diode in the optical head. Magneto-resistive heads as mentioned herein include MR heads and GMR (Giant Magneto-Resistive) heads in which the GMR head effect is applied. Moreover, the transducer is not limited to a MR head or an optical head but can be any active element that requires driving current or driving voltage.
Moreover, the data decoding system in the present invention can be a partial-response system. In a partial-response system, the output signal of the transducer is sampled in synchronism with the clock period. This means that the output signal is utilized only at the instant of sampling, and by synchronizing the driving current for the transducer in the present invention with the clock period for the partial-response system, a sampled signal can be obtained similarly to the conventional case where the transducer output having a time-continuous value is sampled. More specifically, the transducer output of the present invention can be sample-held through a sample-hold circuit, or the like, and then can be inputted to a conventional partial-response demodulator circuit. This way, smaller power consumption and longer lifetime of the transducer are obtained without remarkable design change. Moreover, the partial-response system is a system suitable for high recording density and so fits the purpose of present invention. Incidentally, the PRML system is a commonly applied system for which LSIs are abundantly supplied and is suitable for application to the present invention.
These and other objects, features and advantages of the present invention will become more apparent in view of the following detailed description of the preferred embodiments in conjunction with the drawings.